1. Field of the Invention
This invention relates to a digital signal encoding, editing and transmitting method applied with advantage to recording moving picture signals or acoustic signals on recording media, such as a magneto-optical disc or a magnetic tape, reproducing and displaying the recorded signals on a display, transmitting moving picture signals or acoustic signals over a transmission channel from a transmitter to a receiver in a teleconferencing system, television telephone system or in a broadcasting system, and receiving and displaying the transmitted signals by the receiver.
2. Description of the Related Art
Recently, it has become customary to compress picture signals or acoustic signals using a pre-set reference standard to generate digital bitstream data and to transmit the generated data, optionally in a multiplexed form.
An illustrative example of the above reference standard is the Moving Picture Coding Experts Group (MPEG) standard. The MPEG is an acronym of a research organization for encoding moving pictures for storage of International Organization for Standardization/International Electrotechnical Commission, Joint Technical Committee 1/Sub Committee 29 (ISO/IEC JTC1/SC29). The MPEG1 standard and the MPEG2 standard are ISO11172 and ISO13818, respectively. In these international standards, ISO11172-1 and ISO13818-1 are standardized in connection with multimedia multiplexing, while ISO11172-2 and ISO13818-2 are standardized in connection with video and ISO11172-3 and ISO13818-3 are standardized in connection with audio.
In ISO11172-2 and ISO11172-2, as picture compressing encoding standard, picture signals are encoded (compressed) on a picture basis (on the frame basis or on the field basis) by exploiting picture correlation in the time direction and in the spatial direction.
FIG. 1 shows schematics of an entire device used for encoding (compressing) picture signals for generating an encoded digital bitstream and for decoding the resulting bitstream via a recording media or over a transmission media.
In FIG. 1, picture signals fed to an input terminal 101 are sent to an encoding unit 102 designed in meeting with the above-mentioned MPEG1 (ISO11172-2 ) or MPEG2 (ISO13818-2 ). Although it is each picture that operates as the basis for compression, the size of the encoded (compressed) picture data is not constant. Therefore, a transmission buffer 103 is provided on an output stage and the stored amount in the buffer is fed back to the encoding unit for adjusting the amount of the codes of the picture in order to prevent overflow/underflow of the transmission buffer 103. By adjusting the code volume in this manner, a bitstream is outputted at a stable bit rate from an output stage of the transmission buffer 103.
The output encoded bitstream is sent to a decoding side via a media 105 such as a transmission media or storage media. The decoding side performs an operation reversed from that performed on the encoding side. That is, the bitstream, sent at the constant rate, is received by a reception buffer 106. A decoding unit 107 then reads out the bitstream from the reception buffer 106 in terms of encoded picture data (accessing unit or AC) as a read-out unit. Therefore, readout from the reception buffer 106 becomes intermittent. The read-out picture data are restored by the decoding unit 107 into original picture data which is outputted at an output terminal 108.
FIG. 2 illustrates the operation of the reception buffer (decoder buffer) 106 on the decoding side of FIG. 1. Thus, FIG. 2 illustrates the rate of storage of the picture bitstream entering the reception buffer 106. The reception buffer 106 is fed with picture data at the pre-set constant rate as indicated by a rightwardly rising straight line. Picture data P1, P2, P3, . . . are basically extracted from the buffer at pre-set decoding time points t1, t2, t3, . . . in terms of the encoding unit (accessing unit AU) as a unit of extraction. Thus the buffer occupying amount (amount of storage of the buffer) delineates a serrated trajectory as shown in FIG. 2.
The contents of the prescriptions of the above-mentioned standards ISO11172-2 or ISO13818-2 Annex C refer to encoding without causing overflow or underflow of the decoder buffer. These conditions represent indispensable conditions in recognizing the bitstream as meeting the above-mentioned MPEG conditions.
When handling plural series of picture signals encoded in accordance with the above-mentioned MPEG1 (ISO11172-2) or MPEG2 (ISO13818-2) standards, it is generally difficult to couple plural bitstreams independently encoded in different systems. In particular, so-called seamless junction, that is coupling the pictures continuously at the coupling point without picture standstill (freezing) is practically impossible in the case of bitstreams generated without any limitations.
FIG. 3 shows an instance in which two bitstreams are simply coupled together. In the instance of FIG. 3, decoding of the next second bitstream BS2 is started immediately after the end of display of a temporally previous first bitstream BS1. In general, a queuing time, termed start-up delay, for starting the decoding after storage of a certain volume of data in a buffer, is required at the leading end of a bitstream. Thus, at the junction point J, the decoding/display of the next decoded picture is delayed for a time corresponding to this delay, thus producing the phenomenon in which the displayed frame comes to a standstill for the duration of several frames. Thus, in such case, the so-called seamless reproduction cannot be realized. In FIG. 3, data supply is stopped at a time point x1, and the last picture of the first bit stream BS1 is extracted at the junction point J. At a time point x2, the frame is at a standstill (frozen) for coupling the bitstreams and, at the next time point x3, the first picture of the second bitstream BS2 is extracted.
FIG. 4 shows an instance in which, for seamlessly reproducing bitstreams similar to those shown in FIG. 3, data input to the buffer is caused to occur at a time point earlier by the above-mentioned start-up delay. Since priority is placed on the seamless reproduction in the instance of FIG. 4, the leading picture of the temporary posterior bitstream BS2 is decoded after 1/FR, where FR is the frame rate, as from the decoding of the last picture of the temporarily prior bitstream BS1 (junction point x2 in FIG. 4). In this case, since the bitstream BS2 enters the buffer temporally before decoding of the last picture of the bitstream BS1 (picture extracted at the junction point J), the amount of data storage in the decoder buffer overflows at the time of decoding of the last picture of the bitstream BS1 (junction point J).
Thus, in general, it is practically impossible to realize seamless coupling of bitstreams of picture signals encoded with the conventional encoding method.
It is therefore an object of the present invention to provide a digital signal encoding, editing and transmitting method whereby two bitstreams of picture signals can be coupled seamlessly without overflow or underflow of the decoder buffer or without interposition of still pictures.
In one aspect, the present invention provides a digital signal encoding method of encoding a bitstream of digital signals, wherein, in encoding, pre-set first limitations are imposed on a decoder buffer occupying volume in the vicinity of the trailing end of a first temporally previous bitstream of plural encoded bitstreams desired to be connected together.
According to the present invention, pre-set second limitations are imposed in encoding on a buffer occupying volume in the vicinity of the leading end of a second temporally posterior bitstream of plural encoded bitstreams desired to be connected together.
The first and second limitations are those for securing the decoder buffer volume required for startup of the temporarily posterior second bitstream. More specifically, the first limitations are such that, if the buffer volume necessary for startup of the temporary posterior second bitstream is xcex1, the encoding bit rate is RB and the extraction time from the buffer of the last picture of the first bitstream is T, limitations on the buffer occupying volume are started at a time point of Txe2x88x92(xcex1/RB) and, if a buffer occupying volume for this time point t=0 is BO(t) and the total capacity of the buffer is BF, limitations represented by BO(t)=BFxe2x88x92RBxc3x97t are imposed in encoding the vicinity of the trailing end of the first bitstream.
The second limitations are such that, if the time of extraction from the buffer of the leading picture of the temporarily posterior second bitstream is Txe2x80x2 and the frame rate is RF, the limitations of xcex2 less than xcex1 are imposed in encoding the vicinity of the leading end of the second bitstream, where xcex2 is the buffer occupying volume at a time point Txe2x80x2xe2x88x921xe2x80x2 RF and xcex1 is the buffer volume necessary for startup of the second bitstream.
In coupling the first and second bitstreams, encoded under the specified limitations, a code sequence_end_code appended to the trailing end of the first bitstream, encoded under limitations, is deleted, and
stuffing bits for (xcex1xe2x88x92xcex2), where xcex1 and xcex2 are the buffer volumes specified as above, and for the deleted code sequence_end_code, are appended to the bitstream.